study on hook formation in ultra-low carbon steel …ccc.illinois.edu/s/2005_presentations/12...2...

1

ANNUAL REPORT 2005Meeting date: June 1, 2005

Joydeep SenguptaPostdoctoral Research Associate

& Brian G. Thomas

Department of Mechanical & Industrial EngineeringUniversity of Illinois at Urbana-Champaign

Study on Hook Formation in Ultra-low Carbon Steel Slabs

University of Illinois at Urbana-Champaign • Metals Processing Simulation Lab • J. Sengupta 2

Scale

10 mm0

Oscillation mark

Location of transverse crack

Pitch of OM

Hook-type OMs are generally deeper and can be distinguished visually

Types of oscillation marks

POSCO casting trials on ultra-low carbon slabs (2004):• Typical OM depth = ~0.2 – 0.6 mm• OMs with hooks > 98% on narrow face for different casting conditions (12 out of 13)

= 100% on wide face (7 out of 10) – 70-90% (3 out of 10)

Szekeres, Iron & SteelMaker, 1996

2


Types of hook marks (ultra-low carbon steel)

Entrapped inclusion

Curved hook

Straighthook

Scale

1 mm0

• ~20-35% straight hooks (wide + narrow faces) based on 13 POSCO trials in 2004• OMs with curved hooks are generally deeper (~1.5 times) • Scarfing is usually done to remove hooks & related defects


OM/Hook formation more severe with decreasing carbon content

OMOM

OM

Ultra-low Peritectic Medium

10 mm

200

µ m

Badri et al., Met. Trans. B, 2005Suzuki, CAMP-ISIJ, 1998

• Ultra-low carbon steels have deeper OMs and are more prone to form hooks• Higher solidus (1535 °C ↔ 1500 °C for high carbon steels) &

thinner mushy zone (15 °C ↔ 50 °C for high carbon steels)

3


OM/Hook formation in meniscus region

• Combined effect of heat transfer, solidification, mechanical interactions & fluid flow• Meniscus region: extends to ~10 mm below the metal level• Influencing events last for a very short time-span but occurs periodically


Influential event #1

Heat transfer between shell and mold:Heat is conducted into the mold via liquid flux & re-solidified flux layers• Governed by size & properties of interfacial gap (contact resistance)• Dynamic mold distortion can lower contact resistance locally

Li and Thomas, Private Communication, 2005

-5.0E+05

0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

2.5E+06

0 1 2 3 4 5 6

Time (s)

Hea

t flu

x (W

/m2 )

5.75 inch on mould6.35 mm on casting5.25 inch mm on mould19.05 mm on casting

0.0mm

6.35mm

12.70mm

19.05mm

Casting

5.25"

5.50"

5.75"

6.00"

6.25"

6.50"Mould

Slab

Mold

Meng and Thomas, Met. Trans. B, 2004

4


Periodic rise in heat flux near meniscus

Badri et al., Met. Trans. B, 2005

NST NST

Measurements for ulta-low carbon steel in a laboratory scale simulator at CMU

Rise in heat flux attributed to release of latent heat (meniscus freezing)


Link between periodic rise in heat transfer & OM formation

Badri et al., Met. Trans. B, 2005

Measurements for ulta-low carbon steel in a laboratory scale simulator at CMU

Oscillation marks on a ultra-low carbon steel slab

Periodic rise in heat flux during NST

5



Meniscus shape:• Shape determined by balance of surface tension, pressure & gravity forces

(equilibrium shape can be predicted by Bikerman’s equation)

• Strong function of sulfur content of steel

• Pressure forces due to mold oscillation is transmitted to meniscus via flux layer(both temporal & spatial variations)


Equilibrium meniscus shape

zza22aln

2aza2xx

2222

0−+

+−−=−

)12ln(2

aax0 +−=

g2asteelρ

γ=

x = distance perpendicular to the mold wall in mmZ = distance along the mold wall in mmγ = surface tension between liquid steel and vapor (or flux) in N m-1

ρsteel = density of liquid steel = 7000 kg m-3

g = gravitational acceleration = 9.81 m s-2

From Bikerman, Physical Surfaces, Academic Press, 1970

γ

z

x

Liquid flux

Liquid steel

ρsteel

6


Effect of %S on surface tension

Lee and Morita, ISIJ International, 2002


Meniscus shape – affected by sulfur content

-8

-7

-6

-5

-4

-3

-2

-1

00 5 10 15 20 25 30 35

Horizontal distance, x (mm)

Ver

tical

dis

tanc

e, z

(mm

)

Fe-0.001 pct SFe-0.01 pct SFe-0.1 pct S

γ = ~1.8 N m-1

γ = ~1.6 N m-1

γ = ~1.3 N m-1

Temperature = 1550 oCDensity of liquid steel = 7000 kg m-3

z

x

Liquid flux

Liquid steel

7


Dynamic flow effects at meniscus(due to mold oscillation)

Meniscus is at a sharp angle

to shell tip

Time (s)

Mol

d di

spla

cem

ent (

mm

)

Meniscus shape is flat and angular

Meniscus is smoothly curved

Meniscus is elevated and bulged out

Negative strip

Increasing positive flux pressure:• Brings meniscus close to mold

• Facilitates sudden freezing

Increasing negative flux pressure: • Pushes meniscus away from mold

• Meniscus Bulging

Increasing positive flux pressure:• Brings meniscus close to mold

• Facilitates sudden freezing

Increasing negative flux pressure: • Pushes meniscus away from mold

• Meniscus Bulging

Tanaka and Takatani, CAMP-ISIJ, 1989



Meniscus Freezing:Partial solidification of meniscus can occur in presence of a water-cooled mold (Saucedo, 1991)

• Preservation of instanteneous frozen shape

• Rapid cell growth in the presence of under-cooled liquid (heterogeneous nucleation)

• Incorporated into OM mechanisms(Takeuchi & Brimacombe in 1984)

Yamamura et al., ISIJ International, 1996

8



Fluid flow effects:• Steel jets perturb free surface (standing waves)

• Level fluctuations due to chaotic turbulence & presence of particles/bubbles(buoyancy forces)

• Abrupt changes in operating conditions (e.g. release of nozzle clog)


Local level fluctuations

Lai et al., ISS Steelmaking Conference, 2000

9



Delivery of local superheat:• Distributed via metal jets & follows the fluid flow patterns to reach meniscus• Meniscus typically retains ~30% of total superheat available (no freezing)

0 20 40 60 80 1000

50

100

150

200

250

Casting width: 1300 mmCenter of narrow face

Sup

erhe

at fl

ux (M

W/m

2 )

Distance below meniscus (mm)

Casting speed: 1.2 m/min Casting speed: 1.46 m/min Casting speed: 1.7 m/min

Zhang and Thomas, POSCO Report, 2004



Shell tip deformation:Shell tip may move away or towards the mold surface due to thermal stresses(temperature gradients) and/or mechanical effects

• Solidification mode (large shrinkage is associated with δ→γ transformation)

• Local shell thinning due to presence of air gap & OMs (coupled effect)

• Mechanical interaction between shell tip and solid rim (negative strip period)

• Sticking of shell tip to mold wall in absence of flux (hot tearing)

• Sudden level fluctuation exposing shell interior to flux

10


Level drop effect: shell bending

B.G. Thomas and H. Zhu, Solidification Science & Processing Conference, 1996

• During level drop:Shell interior exposed to fluxLeft edge of shell cools rapidlyShell tip bends away from mold

• After level rise:New shell forms on existing solidBending of shell increased further


OM formation mechanism I

Healing: (i) Sato (Proc. of NOH & BOS Conf.), 1979(ii) Szekeres (Iron & Steel Engineer), 1996

Tearing: Savage and Pritchard (Iron and Steel), 1954

Steel solidifies first against mold wall forming a secondary meniscus& attaches to shell during negative strip time

11


OM formation mechanism II

Shell bending during negative strip

Liquid steel overflow during positive strip

HookOM

Shell distortion (away from mold) & subsequent overflow

(i) Schwerdtfeger and Sha (Met. Trans. B), 2000: Beam bending theory(ii) Thomas and Zhu, 1996: Level drop causes surface depressions(iii)Emi et al. (Proc. of NOH & BOS Conf.), 1978: Solid flux rim effect


OM formation mechanism III

Meniscus freezing during negative strip

Liquid steel overflow during positive strip

HookOM

Meniscus freezing & subsequent overflow

(i) Takeuchi and Brimacombe (Met. Trans. B), 1984: Metallography(ii) Saucedo (SteelMaking Conf. Proc.), 1991: Metallography(iii)Putz et al. (Steel Research), 2003: Heat transfer/Fluid flow model

12


I. Solidification of curved liquid steel meniscus

II. Solidification of steel shell against the mold wall and thensubsequent distortion due to thermal stress / pressure forces

Re-evaluate mechanisms by:• Analysis of hook metallography (POSCO/Postech)• EBSD analysis of hooks• Meniscus shape calculation

• Thermal-stress analysis of initial solidification

Propose new mechanism of hook formation

Two mechanisms for hook formation have been considered in this study


Hook characteristics

Hook depth, D = 1.82 mmHook height, H = 5.37 mmHook length, L = 5.65 mmHook thickness, T = 0.78 mmOM depth, d = 0.228 mmFinal hook angle, θ = 69o

Ultra low carbon steel (0.003% C)POSCO Sample No.: B58077-02-2 No.1-1 (Curved hook)

1 mm

Scale

0

Outline of hook “rapid solidification” region

θ

L H

d

Hook shape (line of hook origin)

Mold surface

Casting direction

D

T

“Meniscus overflow” region

1 mm

Scale

0

Outline of hook “rapid solidification” region

θ

L H

d

Hook shape (line of hook origin)

Mold surface

Casting direction

D

T

“Meniscus overflow” region

13


Hook shapes: Case I

Ultra low carbon steel:0.003C-0.009S-0.08Mn-0.013P-0.039Al-0.047Ti-0.01Ni-0.01Cr-0.01Cu

Casting conditions:Casting speed : 1.42 m/minFrequency: 155 cpmStroke: 6.34 mmSuperheat: 25 oC

POSCO Casting No.: B58077-02Samples taken within ~100mm of slab length

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

0 0.5 1 1.5 2 2.5x (mm)

z (m

m)

Actual shapeBikerman curve


0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

0 0.5 1 1.5 2 2.5

Hook shapes: Case II

Ultra low carbon steel:0.003C-0.009S-0.08Mn-0.013P-0.039Al-0.047Ti-0.01Ni-0.01Cr-0.01Cu

Casting conditions:Casting speed : 1.61 m/minFrequency: 181 cpmStroke: 6.82 mmSuperheat: 27 oC

POSCO Casting No.: B58075-06Samples taken within ~100mm of slab length

z (m

m)

Actual shapeBikerman curve

x (mm)

14


Variations in hook angle, and meniscus shape before & after “meniscus overflow”

Different shapes of meniscus: (a) almost straight to (c) very bentDifferent shapes of overflow region: (a) shallow contact angle, (b) near-vertical contact angle, & (c) strange angle and shape

(a) (b) (c)


A

B

Bubble A entrapped by frozen meniscus butBubble B flows along with the overflowing liquid steel

Debris trapped on both sides of the line of hook origin:

Below the line of hook origin: particles flow up with the steel, reach the meniscus, but do not get entrained into slag layer

Above the line of hook origin: particles flow along with the overflowing liquid steel when it overflows the meniscus

Hooks with particles showing capture before and after “meniscus overflow”

15


Features of a hook tip

Fractured hook tip

Brittle fracture

Actual position of hook and fractured

tip on slab

Original position of fractured tip prior to

brittle fracture

Location of brittle fracture

Location of melting after

fracture


Truncated hooks

Fractured hook tip

Truncated hook

Truncated hook and unmelted fractured tip

Truncated hook (Fractured tip melted away)

16


• Random orientation of dendrites → Heterogeneous nucleation• Fine dendrite arms near origin line → rapid solidification of undercooled liquid• Coarser dendrite arms → Lower temperature gradient

→ long time surrounded by liquid before solidification continued (ripening)• Large sudden change of growth direction → Movement of fractured hook tip

Cell growth near hook

Solidification inward from frozen meniscus

Solidification within the liquid overflow region towards the mold wall


SEM Images

1 mm

100 µmMicrographfrom optical microscope Backscattered SEM image Backscattered SEM image with

70 degrees tilt

200 µm

Hook shape transferred(to scale)

17


Electron Back Scattered Diffraction (EBSD) Map near hook region

100 µm

Grains can be identified on the sample by superimposing an EBSD map containing grain orientation information on the Backscattered SEM image

Backscattered SEM image

200 µm


Quantification of grain misorientation

100 µm 300x

EBSD postprocessor software can provide local grain misorientation data

OVERFLOW REGION

+20o

0o

-1o

+21o+50o

+34o

18


Location of line of hook origin on EBSD image compares well with micrograph

200 µm

OVERFLOW REGION

LINE OF HOOK

ORIGIN

FROZEN MENISCUS


v

xz

y

Solidifyingsteel shell

Position of meniscusz = v x t

CON2D Finite-element model to compute thermal distortion of solidifying steel shell

Simulation Details • Domain Size: 3 mm x 30 mm• 6 noded triangular elements• Mesh resolution: 0.1 mm x 0.5 mm

Assumptions:• Effect of ferrostatic pressure is ignored• No constraint at the mold edge• No mold taper, slag, oscillation, friction• Drop in heat transfer due to air gap ignored• Level is assumed to drop suddenly• No kinetics & undercooling effects

19


Simulation conditions

1.5 s (0.001 s time step size)1.2 s0.8 s (for 16, 8, 5 & 3 mm)1000 °C250 °C1.2 m/min30 °C14.6 °C1533.9 °C1519.3 °C

0.003C-0.009S-0.08Mn-0.013P-0.039Al-0.047Ti-0.01Ni-0.01Cr-0.01Cu

Ultra-low C steel

Sudden level rise atSudden level drop at

Total time for analysis

Tflux

Tmold

Casting speed∆Tsuperheat

∆TTliquidus

Tsolidus

CompositionGrade


Phase fractions

0

0.2

0.4

0.6

0.8

1

1.2

1350 1370 1390 1410 1430 1450 1470 1490 1510 1530 1550

Temperature ( oC)

Phas

e fr

actio

n

delta-ferriteausteniteliquid

Mushy zone

20


Thermophysical properties

25

30

35

40

45

50

750 850 950 1050 1150 1250 1350 1450 1550

Temperature ( oC)

The

rmal

Con

duct

ivity

(W m

-1 K

-1)

0.25

0.5

0.75

1

1.25

1.5

Ent

halp

y (M

J kg

-1)

Thermal conductivityEnthalpy

Tliquidus

Thermal conductivity at T > Tliquidus = 259. 35 Wm-1 (~6.65x)


Mechanical properties

0

10

20

30

40

50

60

70

80

90

100

750 850 950 1050 1150 1250 1350 1450 1550

Temperature ( oC)

You

ng's

Mod

ulus

(GPa

)

-0.02

-0.016

-0.012

-0.008

-0.004

0

0.004

0.008

0.012

0.016

0.02

The

rmal

Lin

ear

Exp

ansi

on (-

)

Young's ModulusThermal linear expansion coefficient

Young's modulus at T > Tliquidus = 100 MPaTliquidus

21


Constitutive behavior

Plastic Strain (%)

Stre

ss(M

Pa)

0 2 4 6 80

5

10

15

20

25 Strain Rate = 1x10-4 1/sec.

Symbols: Experiments [YM WON, Met. Trans. B, 2000]Lines: CON2D (Koslowski III)

1100 oC

1200 oC

1300 oC

Plastic Strain (%)

Stre

ss(M

Pa)

0 2 4 6 80

1

2

3

CON2D, 1425 oCCON2D, 1525 oCExperiment, 1425 oCExperiment, 1525 oC

Strain Rate = 1.4x10-4

Kozlowski III Law for Austenite Power Law for δ-ferrite


Heat Transfer Calculations

Qmold

0

5

10

15

20

25

30

0 0.25 0.5 0.75 1 1.25 1.5

Time (s)

Men

iscu

s lev

el (m

m)

A

B

C

Mold - solidifying shell interface:Qmold = hmold x (Tshell – 250)hmold = 4000 Wm2K-1

Slag – shell interface:Qslag = hslag x (Tshell – Tslag)hslag = 2300 Wm2K-1

Tslag = 1000 oC

Level fluctuation history

Qslag

Qmold

Qmold

Liquid steel

Liquid steel

Liquid steel

Shell Shell Shell

ComputationalDomain

3 x 30 mm

A: Before Level Drop B: During Level Drop C: After Level Rise

Meniscus

Meniscus

Meniscus

Liquid flux

@ 1000 oC

22


Shell deformation during level fluctuation

0

5

10

15

20

25

30

0.7 s 1.1 s

Level drop0.8 s

Level rise1.2 s

Time1.5 s

t = 0.7sNormal solidification & shell growth

t = 1.1sShell cools without growth & bends

t = 1.5sShell grows again with further bending

15001450140013501300125012001150110010501000950900850800750700650600550500

Tem

pera

ture

°C

-30

-25

-20

-15

-10

-5

0

5

10

0 0.5 1 1.5

Time (s)

Men

iscu

s lev

el (m

m)

-10

-5

0

5

10

15

20

25

30

35

0 0.5 1 1.5

Time (s)

Met

al le

vel w

.r.t.

shel

l (m

m)

Leve

l dro

p =

16 m

m

Leve

l ris

e =

24 m

m

Cas

ting

spee

d =

20 m

m/s

∆u = +0.35 mm

∆u = +0.05 mm

Level

Level

Level


Final distorted shape for different level drop distances (0.3s after overflow over shell tip)

Z(m

m)

0 2 40

5

10

15

20

25

30

0 2 40

5

10

15

20

25

30

0 2 40

5

10

15

20

25

30

0 2 40

5

10

15

20

25

30

0 2 40

5

10

15

20

25

3015001450140013501300125012001150110010501000950900850800750700650600550500

No Level Drop∆u = +0.02 mm

3mm drop∆u = +0.02 mm




Temperature °C

Level Drop

Level Drop

Level Drop

Level Drop

LD time: 0.4s

23


Jog

Fredriksson and Elfsberg, Scandinavian J. of Met., 2002

Shell

Jog formation


Comparison between hook and distorted shell shapes

MeniscusOverflow

Deformed shell after level rise

Mold surface

∆u = +0.35 mm

Hook

x (mm)

z (m

m)

1 2

1

2

3

4

5

6Predicted for 16 mm level drop

Predicted for 8 mm level drop

Measured hook shape

Mechanism of hook formation due to shell distortion associated with a

level drop event Shell distortion will affect OM & hook formation during large level drops

POSCO Casting No.: B58077-02

24


Hook depth measured from POSCO samples (2004 data)

0.532.611.6056.044.028225051B30959-55

0.553.211.7961.538.532205151B30959-51

0.421.911.0560.439.629194848B30959-02

0.533.211.1267.332.735175355B30957-01

0.521.581.0365.434.61792626B26848-54

0.622.051.0988.911.12432727B26848-04

0.212.100.9677.122.937114848B31077-54

0.471.921.0682.417.64295152B31077-04

0.381.831.1170.229.833144748B31075-05

0.341.490.7756.843.225194451B31075-04

0.291.600.7375.424.646156162B32793-52

0.441.300.8380.919.13894749B32793-05

0.391.530.8377.122.937114849B32793-02

Min hook depth (mm)

Max hook depth (mm)

Mean hook depth (mm)

% Curved hooks

% Surface hooks

Curved hook #

Surface hook #

Hook #

OM #Sample


Proposed mechanism for hook formation

HOOK FORMATION

CURVED HOOK

STRAIGHT HOOK

Mechanism:Meniscus freezing &

Liquid overflow in the presence of uncooled liquid during negative strip time

Mechanism:Shell distortion during

negative strip time

25


Development of a detailed hook formation mechanism

• Events dictating formation of OMs and hooks are complex, inter-dependent and transient

• Plant experiments cannot reveal detailed steps that lead to final microstructure and morphology

• Development of a comprehensive computational model is a daunting task

• Alternative methodology:- Combine existing modeling results & plant observations- Construct a series of schematics illustrating the mechanism- Will not predict formation of hooks, but will lead to model development


Three hooks observed on a POSCO slab

8 m

m8

mm

Hook #3

Hook #2

Hook #1

I. Steel Composition: C (0.003%) - Mn (0.08%) - Si (0.005%) - P (0.015%) – S (0.01%) - Cr (0.01%) - Ni (0.01%) - Cu (0.01%) – Ti (0.05%) - Al (0.04%) II. Steel Properties: Liquidus temperature (in °C): 1533 Solidus temperature (in °C): 1517 Density of liquid steel (in kg m-3): 7000 Surface tension at 1550 °C (in N m-1): 1.6 III. Slag Properties: Solidification temperature (in °C): 1149 Melting temperature (in °C): 1180 Viscosity at 1300 °C (in Poise): 3.21 IV. Casting conditions: Casting speed: 1.394 m min-1

Frequency of mold oscillation: 174 cpm Stroke of mold oscillation: 5.89 mm Theoretical pitch for oscillation marks (speed/frequency):

8.01 mm

Superheat: 32 °C

26


Objectives

I. Graphically track the positions of the following on a Cartesian space for one mold oscillation cycle:Meniscus, mold, solid/liquid flux interface, shell, OM/Hook mark nos. 2 & 3

II. Using above positions and final morphology of the cast slab sample as templates, events leading to formation of OM & hook mark no. 1 will be identified

Solid flux rim

Liquid flux

channel

Shell

Liquid steel

Meniscus

Cartesian space under scrutiny


Equilibrium shape of meniscus

-8

-7

-6

-5

-4

-3

-2

-1

00 5 10 15 20 25 30 35

Horizontal distance, x (mm)

Ver

tical

dis

tanc

e, z

(mm

)

Fe-0.001 pct STemperature = 1550 oCDensity of liquid steel = 7000 kg m-3

Assumptions: I. Far field metal level remains unpeturbed at all times (z = 0 mm) II. x = 0 mm corresponds to mold wallIII. Dynamic effects active for x < 35 mm

Extends to far-field metal level(towards SEN)

MENISCUS REGION

SHELL

LIQUID STEEL

27


Mold & shell velocity

-90-80-70-60-50-40-30-20-10

0102030405060708090

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Time (s)

Vel

ocity

(mm

/s)

Shell

Oscillating mold

NEGATIVE STRIP

(a)


Mold position

-24-22-20-18-16-14-12-10-8-6-4-20246

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Time (s)

z (m

m)

Mold position w.r.t meniscus level

OM#2 position w.r.t. meniscus level

Meniscus levelz = 0 mm

LABORATORY FRAME

Casting speed = 23.23 mm/s (1.394 m/min)Stroke = 5.89 mmFrequency = 2.90 cps (174 cpm)

NEGATIVE STRIP

(b)

Meniscus shape is assumed to be in equilibrium at tstart = 0 s• Mold acceleration is zero → Inertia force is absent at this instant• Positive flux pressure built-up during NST has been dissipated completely• Hence, only surface tension forces will determine the shape of meniscus

At tstart = 0 s:Position of OM #2:6.4 + 8 = 12.4 mm

Position of OM #3:12.4 + 8 = 20.4 mm

Position corresponds to z = 0 mm & lines up with far-field metal level

28


Profile of solid flux rim

Takeuchi et al., 1983

• Profile of solid rim above the meniscus has been reported in literature, with: Solidus for steel = 1399 oC (actual 1517 oC)Solidus for slag = 1130 oC(actual 1180 oC)Superheat = 5 oC

• The shape has to be modified to correspond to actual superheat of 20 oC


Evolution of shell thickness predicted by CON1D

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50Distance (z) from meniscus level (mm)

Loc

atio

n (x

) pre

dict

ed b

y C

ON

1D (m

m)

LiquidusSolidusShell position

Note: Fraction solid used by CON1D for shell location = 0.5

Cast surface

OM#2

OM#3

OM#1

29


Reduction in shell thickness due to OM

From B.G. Thomas et al., Sensors & Modeling in Mat. Processing, 1997

OM area for OM nos. 1, 2 & 3:(0.2 + 0.16 + 0.24)/0.236 = 2.54 mm2/cm at ~20 mm below meniscus

Based on literature, about ~16% reduction in shell thickness should be observed at each oscillation mark.(Actual for OM#3 = 17%)


Summary of events in one oscillation cycle

-6

-4

-2

0

2

4

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Time (s)

Mol

d po

sitio

n (m

m)

NEGATIVE STRIP

1. Mold moves upwards2. Flux channel above meniscus opens up3. Liquid flux is sucked into this gap4. Suction rate decreases as mold goes to maxima5. Meniscus shape bulges up & away from mold

1. Mold moves downwards2. Flux channel closes up gradually3. Liquid flux is pumped out of above4. But it is also pumped into the flux channel along mold edge5. Meniscus shape flattens out6. Heat flow into mold increases

Freezing of meniscus

Meniscus overflow begins

Rapid growth of new hook

Growth of new hook completed

Brittle fracture of hook tip

1. Mold moves upwards again2. Flux channel opens up again 3. Intake of liquid flux is increased4. Meniscus is pulled towards its equilibrium shape

Meniscus has equilibrium shape

Equilibrium shape of meniscus is restored as a new cycle of mold oscillation begins

New shell begins to grow above hook & OM is formed

95

49


Experimental observations on a Sn-Pballoy/stearic acid based casting simulator

Sudden decrease in ys during NST caused by liquid steel overflow over a

hook & subsequent solidification

Sudden rise in tracer velocity during PST caused by creation of additional space in tracer channel due to OM

formation

Tsutsumi et al., ISIJ International, 2000

96


Summary

I. A new mechanism of hook formation proposed based on:- Analysis of hook micrographs & EBSD images- Meniscus shape predictions by Bikerman equation- Prediction of distorted shell shapes by modifying CON2D- Comparison of hook shapes with both meniscus shapes and

distorted shell shapes(Most hooks match best with meniscus shape)

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Summary

II. Hook formation has been graphically animated & depicts:- Change of meniscus shape during one oscillation cycle- Menicus freezing during negative strip period- Overflow of liquid steel over curved hook (frozen meniscus)

during negative strip period- Hook formation & growth and subsequent fracture of tip

III. The new mechanism can satisfactorily explain the formation of oscillation marks and hooks in ultra-low carbon steel slabsHowever, shell distortion can still play an important role during oscillation mark formation in pertectic steels.

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Shell distortions for different grades without any level fluctuation

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3015101500149014801470146014501440143014201410140013901380137013601350

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Fe-0.04% C∆u = 0.04 mm

Fe-0.13% C∆u = 0.42 mm

Fe-0.47% C∆u = 0.02 mm

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Fe-0.003% C∆u = 0.09 mm

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Acknowledgements

Continuous Casting ConsortiumUniversity of Illinois

Ho-Jung Shin & Go-Gi LeeGraduate Students, POSTECH, South Korea

For hook samples and micrographs

POSCO, South KoreaFor providing samples and data

Natural Sciences & Engineering Research CouncilCanada

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study on hook formation in ultra-low carbon steel …ccc.illinois.edu/s/2005_presentations/12...2...

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